Femtosecond laser-induced formation of submicrometer spikes on a semiconductor substrate

ABSTRACT

The present invention generally provides semiconductor substrates having submicron-sized surface features generated by irradiating the surface with ultra short laser pulses. In one aspect, a method of processing a semiconductor substrate is disclosed that includes placing at least a portion of a surface of the substrate in contact with a fluid, and exposing that surface portion to one or more femtosecond pulses so as to modify the topography of that portion. The modification can include, e.g., generating a plurality of submicron-sized spikes in an upper layer of the surface.

The present application is a continuation of U.S. patent applicationSer. No. 14/073,460 entitled “Femtosecond Laser-Induced Formation ofSubmicrometer Spikes On A Semiconductor Substrate”, filed on Nov. 6,2013, which is a continuation of U.S. patent application Ser. No.13/021,409 filed on Feb. 4, 2011, U.S. Pat. No. 8,598,051, Issued Dec.3, 2013, which is a continuation of U.S. patent application Ser. No.12/235,086, filed on Sep. 22, 2008, U.S. Pat. No. 7,884,446, Issued Feb.8, 2011, which is a divisional of U.S. patent application Ser. No.11/196,929, filed on Aug. 4, 2005, U.S. Pat. No. 7,442,629, Issued Oct.28, 2008, and is a continuation-in-part of Ser. No. 10/950,230 entitled“Silicon-Based Visible And Near-Infrared Optoelectric Devices,” filed onSep. 24, 2004, U.S. Pat. No. 7,057,256, Issued Jun. 6, 2006 and U.S.patent application Ser. No. 10/950,248 entitled “Manufacture ofSilicon-Based Devices Having Disordered Sulfur-Doped Surface Layers,”filed on Sep. 24, 2004, U.S. Pat. No. 7,354,792, Issued Apr. 8, 2008,all of which are herein incorporated by reference in their entirety. Thepresent application is also related to U.S. patent application Ser. No.10/155,429 entitled “Systems and Methods for Light Absorption and FieldEmission Using Microstructured Silicon,” filed on May 24, 2002, U.S.Pat. No. 7,390,689, Issued Jun. 24, 2008, which is also hereinincorporated by reference in its entirety.

FEDERALLY SPONSORED RESEARCH

The invention was made with Government Support under contractDE-FC36-01GO11053 awarded by Department of Energy and under grantNSF-PHY-0117795 awarded by National Science Foundation and. TheGovernment has certain rights in the invention.

BACKGROUND

The present invention is generally directed to methods for processingsemiconductor substrates and the resultant processed substrates, andmore particularly to such methods for modifying the topography of asubstrate's surface.

A number of techniques are known for generating micrometer-sizedstructures on silicon surfaces. For example, quasi-ordered arrays ofconical spikes can be formed on a silicon surface by irradiating it withhigh fluence laser pulses by employing, for example, the methodsdisclosed in the above U.S. patent applications.

There is, however, still a need for enhanced methods that allowgenerating even smaller structures on semiconductor surfaces, andparticularly on silicon surfaces.

SUMMARY

The present invention is directed generally to methods for generatingsubmicron-sized features on a semiconductor surface by irradiating thesurface with short laser pulses. The methods allow modulating the sizesof these features by selecting the irradiation wavelength and/or placinga surface portion to be irradiated in contact with a fluid. Theinvention can provide formation of features that are substantiallysmaller in size than those generated by previous techniques. Thegenerated features can be, for example, in the form of substantiallycolumnar spikes, each of which extends from a base to a tip, thatprotrude above the surface. In many embodiments, the average height ofthe spikes (i.e., the average separation between the base and the tip)can be less than about 1 micron, and the spikes can have an averagewidth—defined, for example, as the average of the largest dimensions ofcross-sections of the spikes at half way between the base and thetip—that ranges from, about 100 nm to about 500 nm (e.g., in a range ofabout 100 nm to about 300 nm).

In one aspect, the present invention provides a method of processing asemiconductor substrate that includes placing at least a portion of asurface of the substrate in contact with a fluid, and exposing thatportion to one or more short laser pulses so as to modify itstopography. The laser pulses can be selected to have pulse widths in arange of about 50 femtoseconds to about a few nanoseconds, and morepreferably in a range of about 100 femtoseconds to about 500femtoseconds.

In a related aspect, the laser pulses are selected to have energies in arange of about 10 microjoules to about 400 microjoules (e.g., 60microjoules), and fluences in a range about 1 kJ/m² to about 30 kJ/m²,or from about 3 kJ/m² to about 15 kJ/m², or from about 3 to about 8kJ/m². The central wavelength of the pulses can be selected to be lessthan about 800 nm, and preferably in a range of about 400 nm to lessthan about 800 nm. The number of pulses applied to each location of thesurface can be, e.g., in a range of about 1 to about 2500.

In many embodiments, utilizing irradiation wavelengths that are lessthan about 800 nm, e.g., 400 nm, and/or placing the irradiated portionin contact with the liquid (e.g., water) can lead to formation ofsub-micron-sized features over the substrate's surface.

In further aspects, in the above method, the fluid can be selected to beany suitable polar or non-polar liquid. Some examples of such liquidsinclude, without limitation, water, alcohol and silicon oil. Further,the semiconductor substrate can be selected to suit a particularapplication. By way of example, in some embodiments, the substrate canbe an undoped or doped silicon wafer (e.g., an n-doped silicon wafer).

In another aspect, the invention provides a semiconductor substrate thatincludes a surface layer having at least a portion that exhibits anundulating topography characterized by a plurality of submicron-sizedfeatures having an average height less than about 1 micrometer and anaverage width in a range of about 100 nm to about 500 nm, and preferablyin a range of about 100 nm to about 300 nm. The substrate can be anysuitable semiconductor substrate, e.g., silicon.

In related aspects, the surface layer has a thickness in a range ofabout 100 nm to about 1 micrometer and the submicron-sized featurescomprise spikes each of which extends from a base to tip separated fromthe base by a distance that is less than about 1 micron. For example,the spikes can protrude above the semiconductor surface by a distance ina range of about 100 nm to about 300 nm.

In another aspect, a method of processing a silicon substrate isdisclosed that includes irradiating a portion of a semiconductor surfacewith one or more femto-second laser pulses having a center wavelength ina range of about 400 nm to less than about 800 nm so as to generate aplurality of submicron-sized spikes within an upper layer of thatsurface. The spikes can have an average height less than about 1micrometer and an average width in a range of about 100 nm to about 590nm.

In a related aspect, in the above method, the irradiation of the surfaceportion is performed while that portion is in contact with a fluid. Byway of example, the fluid can include a polar or non-polar liquid, or agas, e.g., one having an electron-donating constituent.

In a related aspect, the above method further calls for disposing asolid substance having an electron-donating constituent on the surfaceportion that is in contact with the fluid prior to its irradiation bythe laser pulses. For example, sulfur powder can be applied to thesurface followed by disposing a layer of fluid (e.g., having a thicknessin a range of about 1 mm to about 20 mm) on the surface. Subsequently,the surface can be irradiated by the laser pulses so as to generate thespikes within an upper layer of the surface and also generate sulfurinclusions in that layer.

In another aspect, the fluid that is in contact with the substrate'ssurface comprises an aqueous solution, e.g., one containing anelectron-donating constituent. By way of example, the liquid cancomprise sulfuric acid.

In a further aspect, a method of processing a semiconductor substrate isdisclosed that includes disposing a solid substance having anelectron-donating constituent on at least a portion of a surface of thesubstrate, and irradiating that surface portion with one or more pulseshaving pulse widths in a range of about 50 fs to about 500 fs so as togenerate a plurality of inclusions containing the electron-donatingconstituent in a surface layer of the substrate. The electron-donatingconstituent can be, for example, a sulfur-containing substance.

Further understanding of various aspects of the invention can beobtained by reference to the following detailed description inconjunction with the associated drawings, which are described brieflybelow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart depicting various steps in one exemplaryembodiment of a method according to the teachings of the invention.

FIG. 2 schematically depicts a semiconductor substrate on a surface ofwhich a plurality of submicron-sized spikes are formed in accordancewith the teachings of the invention,

FIG. 3 is a flow chart depicting steps in another exemplary embodimentof a method according to the teachings of the invention for changingtopography of a semiconductor surface,

FIG. 4 is a schematic diagram of an exemplary apparatus suitable for usein practicing the processing methods of the invention,

FIGS. 5A and 5B are scanning electron micrographs of silicon spikesformed on a silicon surface viewed at 45° angle relative to a normal tothe surface, formed by placing the surface in contact with distilledwater and irradiating it with 100-fs, 400-nm, 60-μJ laser pulses,

FIG. 5C is a scanning electron micrograph of the spikes shown in FIGS.5A-5B viewed from the side,

FIG. 6A is another scanning electron micrograph of silicon spikes formedin distilled water by irradiating a silicon surface with femtosecondpulses before etching the surface to remove an upper silicon oxidelayer,

FIG. 6B is a scanning electron micrograph of the silicon spikes of FIG.6A after having been subjected to etching in HF, and

FIGS. 7A-7J are scanning electron micrographs of a silicon surfaceirradiated while in contact with distilled water by an increasing numberof laser pulses (the width of the irradiated area is approximately 50μm).

DETAILED DESCRIPTION

The present invention generally provides semiconductor substrates havingsurfaces that exhibit submicron-sized structures, and methods forgenerating such structures. In many embodiments, the submicron-sizedstructures are generated by irradiating a semiconductor substrate'ssurface with ultra short laser pulses (e.g. femtosecond pulses) whilemaintaining the surface in contact with a fluid (e.g., water). Exemplaryembodiments of the invention are discussed below.

With reference to a flow chart 10 of FIG. 1, in one exemplar embodimentof a method according to the teachings of the invention for processing asemiconductor substrate, in a step 12, at least a portion of thesubstrate surface is placed in contact with a fluid, for example, bydisposing a layer of the fluid over that portion. In another step 14,the substrate portion that is in contact with the fluid is exposed toone or more short laser pulses so as to modify its surface topography.The laser pulses can have pulse widths in a range of about 100 fs toabout a few ns, and more preferably in a range of about 100 fs to about500 fs. In this exemplary embodiment, the center wavelength of thepulses is chosen to be about 400 nm. More generally, wavelengths in arange of about 400 nm to less than about 800 nm can be employed. Thepulse energies can be in a range of about 10 microjoules to about 400microjoules, and preferably in a range of about 60 microjoules to about100 microjoules.

The modification of the surface topography can include generatingsubmicron-sized features in an upper surface layer of the substrate. Forexample, the submicron-sized features can include a plurality ofmicrostructured spikes, e.g., columnar structures extending from thesurface to a height above the surface. FIG. 2 schematically depicts aplurality of such features (also referred to herein as spikes) 16 formedon a semiconductor substrate surface 18. Each spike can be characterizedby a height and a width (the spikes are shown only for illustrativepurposes and are not intended to indicate actual density, size orshape). For example, a spike 16 a has a height H defined as theseparation between its base 20 and its tip 22, and a width defined by adiameter D of a cross-section, e.g., one substantially parallel to thesubstrate surface, at a location half way between the base and the tip.In ease of irregularly shaped spikes, the width can correspond, e.g., tothe largest linear dimension of such a cross-section of the spike. Inmany embodiments, the submicron-sized features exhibit an average heightof about 1 micrometer (e.g., a height in a range of about 200 nm about 1micrometer) and an average width in a range of about 100 nm to about 500nm, or in a range of about 100 nm to about 300 nm.

In general, the fluid is selected to be substantially transparent toradiation having wavelength components in a range of about 400 nm toabout 800 nm. Further, the thickness of the fluid layer is preferablychosen so as to ensure that it would not interfere with the laser pulses(e.g., via excessive self-focusing of the pulses) in a manner that wouldinhibit irradiation of the substrate surface. While in this embodimentwater is selected as the fluid, in other embodiments other fluids, suchas alcohol or silicon oil, can be employed.

In some embodiments, at least a portion of the substrate can be placedin contact with an aqueous solution having an electron-donatingconstituent. For example, a solution of sulfuric acid can be applied toat least a portion of the substrate followed by irradiating that portionwith short pulses (e.g., femto-second pulses) to not only cause a changein surface topography in a manner described above but also generatesulfur inclusions within a surface layer of the substrate.

Referring to a flow chart 24 of FIG. 3, in another embodiment, initiallya solid compound, e.g., sulfur powder, is applied to at least a portionof a semiconductor substrate surface (e.g., a surface of a siliconwafer) (step 26). Subsequently, in a step 28, that surface portion isplaced in contact with a fluid (e.g., water) and is irradiated (step 30)by one or more short laser pulses (e.g., pulses with pulse widths in arange of about 100 fs to about a few ns, and preferably in a range ofabout 100 fs to about 500 fs) so as to cause a change in topography ofthat portion. Similar to the previous embodiment, the pulse energies canbe chosen to be in a range of about 10 microjoules to about 400microjoules (e.g., 10-150 microjoules).

FIG. 4 schematically depicts an exemplary apparatus 32 suitable forperforming the above methods of processing a semiconductor substrate.The apparatus 32 includes a Titanium-Sapphire (Ti:Sapphire) laser 34that generates laser pulses with a pulse width of 80 femtoseconds at 800mm wavelength having an average power of 300 mW and at a repetition rateof 95 MHz. The pulses generated by the Ti:Sapphire laser are applied toa chirped-pulse regenerative amplifier 36 that, in turn, produces 0.4millijoule (mJ), 100 femtosecond pulses at a wavelength of 800 nm and ata repetition rate of 1 kilohertz.

The apparatus 32 further includes a harmonic generation system 38 thatreceives the amplified pulses and doubles their frequency to produce 140microjoule, 100-femtosecond second-harmonic pulses at a wavelength of400 nanometers. The harmonic generation system can be of the typecommonly utilized in the art. For example, it can include a lens 38 afor focusing the incoming pulses into a doubling crystal 38 b to cause aportion of the incoming radiation to be converted into second-harmonicpulses. A dichroic mirror 38 c can direct the second-harmonic pulses toa lens 40, and a beam stop 38 d can absorb the portion of the radiationthat remains at the fundamental frequency.

The lens 40 focuses the second-harmonic pulses onto a surface of asample 42 (e.g., a silicon wafer) disposed on a 3-dimensionaltranslation system 44 within a vacuum chamber 46. A glass liquid cell 48is coupled to the stage over the sample so as to allow a sample surfaceto have contact with the fluid (e.g., water) contained within the cell.The three-dimensional stage allows moving the sample relative to thelaser pulses for exposing different portions of the surface toradiation. The vacuum chamber can be utilized to pump out air bubbles inthe fluid. Alternatively, the processing of the sample can be performedwithout utilizing a vacuum chamber.

To Illustrate the efficacy of the teachings of the invention and onlyfor illustrative purposes, submicrometer-sized silicon spikes weregenerated in surface layers of silicon wafers submerged in water byirradiating those surfaces with 400-nm, 100-fs laser pulses. Forexample, a Si (111) wafer was initially cleaned with acetone and rinsedin methanol. The wafer was placed in a glass container, such as thecontainer 48 described above, that was filled with distilled water andmounted on a three-axis stage. The silicon surface in contact with thewater was irradiated by a 1-KHz train of 100-fs, 60-microjoule pulses ata central wavelength of 400 nm generated by a frequency-doubled,amplified Ti:Sapphire laser, such as that described above. A fastshutter was utilized to control the number of laser pulses incident onthe silicon surface. The laser pulses were focused by a 0.25-mfocal-length lens to a focal point about 10 mm behind the siliconsurface. The pulses traveled through about 10 mm of water beforestriking the silicon surface at normal incidence. The spatial profile ofthe laser spot at the sample surface was nearly Gaussian characterizedby a fixed beam waist of about 50 microns. To correct for chirping ofthe laser pulses in the water and to ensure minimum pulse duration atthe silicon surface, the pulses were pre-chirped to obtain the lowestpossible damage threshold at that surface. The results, however, did notdepend strongly on the chirping of the laser pulses.

During sample irradiation, the irradiated sample surface was monitoredwith an optical imaging system having a spatial resolution of about 5microns. It was observed that irradiation cause formation ofmicrometer-sized water bubbles at the silicon-water interface. After asingle pulse, two or three microbubbles were generated; afterirradiation with trains of laser pulses thousands of bubbles weregenerated. It was also observed that some bubbles at times wouldcoalesce to form larger ones, which would adhere to the silicon surface.These larger bubbles were removed by shaking the cell.

FIGS. 5A, 5B, and 5C present electron micrographs of the silicon surfaceafter irradiation with one thousand laser pulses, showing formation of aplurality of spikes on the surface. The spikes have a substantiallycolumnar shape with a typical height of about 500 nm and a typicaldiameter of about 200 nm. They protrude up to about 100 nm above theoriginal surface of the wafer (FIG. 1C). The shape of the spikes is morecolumnar than the conical spikes that can be formed in the presence ofSF₆ as disclosed in the above-referenced patent applications.

The chemical composition of the uppermost 10 nm of the silicon surfacelayer having the spikes was determined by employing X-ray photoelectronspectroscopy (XPS). The XPS spectra showed that this layer is composedof about 83% SiO₂ and about 17% silicon. The wafer was etched in 5% HFfor about 15 minutes to remove the SiO₂ layer (about 20 nm in thickness)while leaving the underlying unoxidized Si intact. A comparison of theelectron micrographs of the spikes before etching (FIG. 6A) with thoseobtained after etching (FIG. 6B) showed that the etching process hadreduced the width of the spikes by about 40 nm and had rendered theirsurfaces smoother. After etching, no SiO₂ was detected in the X-rayphotoelectron spectra of the sample, thereby indicating that theinterior of the spikes consisted of silicon and that the spikes werecovered prior to etching by an oxide layer that was at most about 20 nmthick.

To study the development of the spikes, silicon samples were irradiated,with different numbers of laser pulses. FIG. 7A-7J show a series ofscanning electron micrographs of the surface of a silicon substrateirradiated with an increasing number of femtosecond laser pulses whilein contact with water, in a manner described above. The images only showthe central portion of the irradiated area. As shown in FIG. 7A, asingle laser pulse forms surface structures resembling ripples on aliquid surface with a wavelength of about 500 nm. Lower magnificationmicrographs (not shown here) indicate that the irradiated regiontypically contains two or three of these ripple-like structures. Withoutbeing limited to any particular theory, each ripple structure is likelyto correspond to one of the microbubbles that were observed afterirradiation.

Referring to FIG. 7B, after two pulses, the surface shows overlappingripple structures. As the number of laser pulses is increased from 5 to20 (FIGS. 7C, 7D and 7E), the silicon surface roughens from theinteraction of many ripple structures. After exposure to 50 laser pulses(FIG. 7F), the surface is covered with submicrometer bead-likestructures, which then evolve into spikes as the number of pulses isfurther increased as shown in FIGS. 7G, 7H, 7I and 7J corresponding,respectively, to irradiating the surface with 100, 200, 300 and 400laser pulses. The average separation of the resulting spikes is roughly500 nm and substantially equal to the wavelength of the initial ripplestructures.

The above silicon spikes prepared in water are one to two orders ofmagnitude smaller than spikes that can be generated in a siliconsubstrate exposed to laser pulses in presence of a gas, such as thosedescribed in the above-referenced copending patent applications. Thisremarkable size difference suggests different formation mechanisms forthe two types of spikes.

Without being limited to any particular theory, it is noted that when a400-nm laser pulse interacts with the silicon surface, most of the lightis absorbed by a silicon layer tens of nanometers thick near thesilicon-water interface. The absorption of intense light in such a thinsilicon layer can excite a plasma at the silicon-water interface, whichcan then equilibrate with the surrounding water and silicon, leavingbehind a molten silicon layer on the surface. The molten layer cansolidify before the next laser pulse arrives. Due to the hightemperature of the plasma, some of the water can vaporize or dissociate,thereby generating bubbles at the silicon-water interface. The largebubbles that were observed after irradiation in the above experimentsremain in the water for days, thus suggesting that they consistprimarily of gaseous hydrogen and oxygen rather than water vapor.

Again, without being limited to any particular theory, several possiblemechanisms can be considered by which the bubbles may produce thewave-like structures shown in FIGS. 7A-7G. Diffraction of the laser beamby the bubbles may produce rings of light intensity on the siliconsurface, or the heat of vaporization and dissociation required to form abubble at the silicon-water interface may cool the silicon surfacelocally, exciting a capillary wave in the molten silicon throughMarangoni flow. The latter is the most likely formation mechanism forthe structures observed after a single pulse; those structures cannot beformed by diffraction from a laser-induced bubble, as the pulse durationis only 100 fs, and the observed wave-like structures can be severalmicrometers in diameter. A micrometer-sized bubble requires much longerthan 100 fs to form and therefore cannot diffract the first pulse.

Roughness on the silicon surface can cause an uneven absorption of thelaser pulse energy across the surface. The resulting non-uniformtemperature of the surface can produce a random arrangement of bubbles.Silicon-water has a contact angle more than 45° making a gaseous layerbetween the silicon and water unstable and leading to the formation ofbubbles. The vaporization and dissociation of the bubbles can removethermal energy from the molten silicon surface just below the bubbles,causing the surface to cool rapidly. Because the surface tension ofliquid silicon decreases with increasing temperature, the surroundinghot liquid silicon flows toward the cooled region, deforming thesurface. This deformation can then excite a circular capillary wave atthe liquid-silicon surface. Superposition of ripple-structures caused bymultiple laser pulses can then produce the randomly distributedsubmicrometer beads that appear after 20 laser pulses (See FIGS. 7F-7J).These beads subsequently sharpen into spikes through preferentialremoval of material around the beads by laser-assisted etching.

As noted above, the morphology and sizes of the above spikes generatedin a silicon surface by exposing it to femtosecond laser pulses while incontact with water can be different than those observed for spikesgenerated by irradiating a silicon surface with femtosecond pulses inpresence of a gas, such as SF₆. The early stage of submicrometer spikeformation in water can be different from that in gaseous SF₆, while thelater stages can be similar. In SF₆, straight submicrometer-sized ripplestructures first form on the silicon surface, then coarser,micrometer-scale ridges form on top of (and perpendicular to) theripples. Next, the coarsened layer breaks up into micrometer-sizedbeads, and finally the beads evolve into spikes through etching. In bothSF₆ and water, the length scale of the final structures is set by thearrangement of beadlike structures that form after roughly 10-20 pulses,and this length scale appears to be determined by capillary waves in themolten silicon. The much smaller size of the spikes formed in water islikely to be due to a difference in capillary wavelength in the twocases.

The molten silicon layer is expected to solidify much faster in waterthan in SF₆, as the thermal conductivity and heat capacity of liquidwater are much greater than those of gaseous SF₆. The dispersionrelation for capillary waves in a shallow layer of molten siliconindicates that decreasing the lifetime of the molten layer should alsodecrease the longest allowed capillary wavelength. Using a simple modelthat neglects the effects of ablation and cooling by heat transfer tothe environment to calculate the lifetime and depth of the liquid layer,it was found that the longest allowed capillary wavelength is about 1micron. Because the lifetime is certainly reduced by the flow of heat tothe surrounding water in the experiments presented above, the longestallowed wavelength should be less than 1 micron, which is in agreementwith submicrometer spike separation observed here.

In some embodiments, rather than utilizing a fluid, a solid substancehaving an electron-donating constituent (e.g., a sulfur powder) isdisposed on at least a portion of a surface of semiconductor substrate,e.g., a silicon wafer. That surface portion is then irradiated with oneor more pulses having pulse widths in a range of about 50 fs to about500 fs so as to generate a plurality of inclusions containing theelectron donating constituent in a surface layer of the substrate.

Those having ordinary skill in the art will appreciate that variouschanges can be made to the above embodiments without departing from thescope of the invention.

What is claimed is:
 1. A method for processing a silicon substrate,comprising: disposing a solid substance on a surface of a siliconsubstrate, wherein said solid substance comprises an electron-donatingconstituent, placing said surface in contact with a liquid such that atleast a portion of said liquid is in contact with said solid substance,subsequently, irradiating said surface of the substrate with a pluralityof laser pulses having a pulse width in a range of about 50 femtosecondsto about 500 femtoseconds in presence of said liquid on said surface soas to incorporate said electron-donating constituent of said solidsubstance as a plurality of inclusions within an upper layer of thesubstrate.
 2. The method of claim 1, wherein said electron-donatingconstituent is a sulfur-containing substance such that said inclusionscomprise sulfur.
 3. The method of claim 1, wherein said solid substancecomprises sulfur powder.
 4. The method of claim 1, wherein said liquidcomprises an aqueous solution.
 5. The method of claim 1, wherein saidliquid comprises any of water, alcohol, sulfuric acid and silicon oil.6. The method of claim 1, wherein said semiconductor substrate is any ofa doped and undoped silicon wafer.
 7. The method of claim 1, whereinsaid laser pulses have a wavelength of less than or about 800 nm.
 8. Themethod of claim 1, wherein said upper layer has a thickness in a rangeof about 100 nm to about 1 micrometer.
 9. The method of claim 1, whereinthe laser pulses are applied to a plurality of surface locations andwherein a number of the laser pulses applied to each surface location isin a range of 1 to about
 2500. 10. The method of claim 1, whereinplacing said surface in contact with said liquid comprises disposing alayer of said liquid having a thickness in a range of about 1 mm toabout 20 mm on said surface.
 11. The method of claim 1, wherein each ofsaid laser pulses has an energy in a range of about 10 microjoules toabout 400 microjoules.